Holography allows a three-dimensional object or a moving 3D scene to be recorded and optically reproduced using wave-optical methods. The 3D scene is encoded on a light modulator which serves as a carrier medium. Due to the illumination with light waves which are capable of generating interference, each point of the encoded 3D scene forms a point of origin of light waves interfering with each other and which, as a resultant light wave front, spatially reconstruct the 3D scene as if it was generated by light propagating from a real object in space. The holographic reconstruction of the object or the 3D scene is preferably realised with the help of a projection device and/or an optical reconstruction system by illuminating a carrier medium with normally sufficiently coherent light.
In this document, the 3D scene is reconstructed in a holographic reproduction device with an observer window, which is a viewing zone in an observer region. The size of the observer window in front of a display means is defined; it is typically as large as an eye pupil. This is why it is referred to as the eye position, in which an observer eye can be situated, and from which the observer can see the reconstruction of the 3D scene.
Seen from the wave-optical point of view, an observer window is formed either by a direct or an inverse Fourier transform or Fresnel transform of a hologram encoded on a carrier medium, or by the imaging of a wave front encoded on a carrier medium in a plane of an observer region, where the observer window comprises only one diffraction order of a periodical reconstruction. The plane may be a focal plane of a focusing means or the image plane of a light source. The hologram or the wave front are computed from the 3D scene such that, within the one diffraction order which is used as the viewing zone, cross talk between the observer eyes is prevented, which would typically occur in reconstructions when using light modulators. In conjunction with an arrangement or a method for suppressing higher diffraction orders, 3D scenes can be consecutively presented in multiplex process to a left and to a right eye of an observer without any cross talk. Moreover, a multiplex process with the aim to serve multiple persons only becomes possible then.
Carrier or recording media for holograms and complex wave fronts of a 3D scene include spatial light modulators, such as LCD, LCoS etc., which modulate the phase and/or amplitude of incident light. The refresh rate of the carrier medium must be sufficiently high in order to be able to represent moving 3D scenes.
The values which are encoded into regularly arranged pixels on the carrier medium can be originated from a real object or be a computer-generated hologram (CGH).
The observer can view the reconstruction of the 3D scene by looking directly on to the carrier medium. In this document, this arrangement is referred to as direct-view display. Alternatively, the observer can look on to a screen on to which either an image or a transform of the values encoded on the carrier medium is projected. In this document, this arrangement is referred to as projection display.
Both the screen in the projection display and the carrier medium in the direct-view display are meant by the term ‘screen’ below hologram is because of the effects of diffraction only possible within one periodicity interval of the reconstruction of a wave front, said periodicity interval being defined by the resolution of the carrier medium. The reconstruction is typically repeated showing irregularities in adjacent periodicity intervals.
Disturbing patterns, which are also known as speckle patterns or granulation, occur when using coherent laser light for illuminating a light modulator. Speckle can be described as a granulation-like interference pattern which is created by interference of multiple light waves with statistically irregularly distributed phase differences.
The reconstruction of a hologram is adversely affected by the speckle patterns. The 3D scene is typically discretely scanned for hologram computation, because it can only be recorded discretely on the carrier medium. Certain encoding methods, where information of the 3D scene is recorded in a suitable manner on the carrier medium, generally make possible a reconstruction where the reconstruction is fully identical to the scanned object at the positions of the scan points. The physical reconstruction results in a continuous gradient, also between the scan points. Deviations from the light intensity gradient in the object occur between the scan points, so that the reconstruction contains speckle patterns, which reduce the quality of the reconstruction. This is in particular the case when computing the hologram with a random phase of the object points, which is, however, advantageous for certain other reasons.
Reducing the speckle patterns in the reconstruction of the 3D scene can be realized by temporal or/and spatial averaging, where the reconstruction is generated from values of a 3D scene encoded on an external carrier medium or from hologram values which are computed in another suitable way. The eye of the observer always averages multiple reconstructions presented to him with different speckle patterns, resulting in a perceivable reduction of this disturbance.
In document DE 195 41 071 A1, for example, a rotating rectangular glass plate is put into the optical path for a time average of the granulation when checking a hologram. The speckles do not appear disturbing anymore because the glass plate rotates at a frequency which is adapted to that of a detector. However, such a method can only be applied for reducing a two-dimensional, plane speckle pattern, where the diffusing screen must be disposed in the plane of the speckle pattern.
A known method of time averaging in order to reduce speckle patterns of a 3D scene is that the 3D scene is to compute with a given number of different random phases, and to represent the respective holograms on a carrier medium one after another at a fast pace. Due to the multiple hologram computations, the computational load increases considerably and the refresh rate of the carrier medium would also have to increase significantly when representing the holograms, which is undesired.
As regards spatial averaging, it is generally known from the literature to divide a carrier medium into multiple independent regions, where repetitions of subholograms which are computed from the same object, but with different object phases, are written next to each other and/or below each other. The eye of the observer averages different speckle patterns of the individual reconstructions of the computed sub-holograms generated with a Fourier transformation or Fresnel transformation, so that the resulting speckle pattern appears weakened.
However, this method cannot be applied to a holographic display with an observer window, as described by the applicant in document DE 103 53 439 A1 and on which this document is based. A complex-valued light distribution of the diffraction image of an object, e.g. a 3D scene, is computed in the observer window of an observer plane. Transformations of individual object planes, into which the 3D scene is virtually sliced, are realised and added in the observer window in order to achieve this. The transformations correspond with the optical propagation of light between the sliced object planes and the observer plane comprising the observer window. This method has the effect that each object point is assigned with a confined localised section on a screen, to which the information for the reconstruction of this point is written. This is necessary to allow a correct reconstruction from the observer window.
Encoding multiple sub-holograms, which are computed from the 3D scene next to each other and/or below each other on the screen, as suggested in the prior art, would have the effect that the hologram values which correspond to an object point are repeated in different sections on the screen. This is not possible though in conjunction with the principle of making visible the reconstructed 3D scene from the observer window. It is a general disadvantage of a spatial repetition of subholograms that the resolution of each individual sub-hologram is reduced in a given carrier medium.